xref: /openbmc/linux/fs/eventfd.c (revision 4800cd83)
1 /*
2  *  fs/eventfd.c
3  *
4  *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
5  *
6  */
7 
8 #include <linux/file.h>
9 #include <linux/poll.h>
10 #include <linux/init.h>
11 #include <linux/fs.h>
12 #include <linux/sched.h>
13 #include <linux/kernel.h>
14 #include <linux/slab.h>
15 #include <linux/list.h>
16 #include <linux/spinlock.h>
17 #include <linux/anon_inodes.h>
18 #include <linux/syscalls.h>
19 #include <linux/module.h>
20 #include <linux/kref.h>
21 #include <linux/eventfd.h>
22 
23 struct eventfd_ctx {
24 	struct kref kref;
25 	wait_queue_head_t wqh;
26 	/*
27 	 * Every time that a write(2) is performed on an eventfd, the
28 	 * value of the __u64 being written is added to "count" and a
29 	 * wakeup is performed on "wqh". A read(2) will return the "count"
30 	 * value to userspace, and will reset "count" to zero. The kernel
31 	 * side eventfd_signal() also, adds to the "count" counter and
32 	 * issue a wakeup.
33 	 */
34 	__u64 count;
35 	unsigned int flags;
36 };
37 
38 /**
39  * eventfd_signal - Adds @n to the eventfd counter.
40  * @ctx: [in] Pointer to the eventfd context.
41  * @n: [in] Value of the counter to be added to the eventfd internal counter.
42  *          The value cannot be negative.
43  *
44  * This function is supposed to be called by the kernel in paths that do not
45  * allow sleeping. In this function we allow the counter to reach the ULLONG_MAX
46  * value, and we signal this as overflow condition by returining a POLLERR
47  * to poll(2).
48  *
49  * Returns @n in case of success, a non-negative number lower than @n in case
50  * of overflow, or the following error codes:
51  *
52  * -EINVAL    : The value of @n is negative.
53  */
54 int eventfd_signal(struct eventfd_ctx *ctx, int n)
55 {
56 	unsigned long flags;
57 
58 	if (n < 0)
59 		return -EINVAL;
60 	spin_lock_irqsave(&ctx->wqh.lock, flags);
61 	if (ULLONG_MAX - ctx->count < n)
62 		n = (int) (ULLONG_MAX - ctx->count);
63 	ctx->count += n;
64 	if (waitqueue_active(&ctx->wqh))
65 		wake_up_locked_poll(&ctx->wqh, POLLIN);
66 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
67 
68 	return n;
69 }
70 EXPORT_SYMBOL_GPL(eventfd_signal);
71 
72 static void eventfd_free_ctx(struct eventfd_ctx *ctx)
73 {
74 	kfree(ctx);
75 }
76 
77 static void eventfd_free(struct kref *kref)
78 {
79 	struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
80 
81 	eventfd_free_ctx(ctx);
82 }
83 
84 /**
85  * eventfd_ctx_get - Acquires a reference to the internal eventfd context.
86  * @ctx: [in] Pointer to the eventfd context.
87  *
88  * Returns: In case of success, returns a pointer to the eventfd context.
89  */
90 struct eventfd_ctx *eventfd_ctx_get(struct eventfd_ctx *ctx)
91 {
92 	kref_get(&ctx->kref);
93 	return ctx;
94 }
95 EXPORT_SYMBOL_GPL(eventfd_ctx_get);
96 
97 /**
98  * eventfd_ctx_put - Releases a reference to the internal eventfd context.
99  * @ctx: [in] Pointer to eventfd context.
100  *
101  * The eventfd context reference must have been previously acquired either
102  * with eventfd_ctx_get() or eventfd_ctx_fdget().
103  */
104 void eventfd_ctx_put(struct eventfd_ctx *ctx)
105 {
106 	kref_put(&ctx->kref, eventfd_free);
107 }
108 EXPORT_SYMBOL_GPL(eventfd_ctx_put);
109 
110 static int eventfd_release(struct inode *inode, struct file *file)
111 {
112 	struct eventfd_ctx *ctx = file->private_data;
113 
114 	wake_up_poll(&ctx->wqh, POLLHUP);
115 	eventfd_ctx_put(ctx);
116 	return 0;
117 }
118 
119 static unsigned int eventfd_poll(struct file *file, poll_table *wait)
120 {
121 	struct eventfd_ctx *ctx = file->private_data;
122 	unsigned int events = 0;
123 	unsigned long flags;
124 
125 	poll_wait(file, &ctx->wqh, wait);
126 
127 	spin_lock_irqsave(&ctx->wqh.lock, flags);
128 	if (ctx->count > 0)
129 		events |= POLLIN;
130 	if (ctx->count == ULLONG_MAX)
131 		events |= POLLERR;
132 	if (ULLONG_MAX - 1 > ctx->count)
133 		events |= POLLOUT;
134 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
135 
136 	return events;
137 }
138 
139 static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
140 {
141 	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
142 	ctx->count -= *cnt;
143 }
144 
145 /**
146  * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
147  * @ctx: [in] Pointer to eventfd context.
148  * @wait: [in] Wait queue to be removed.
149  * @cnt: [out] Pointer to the 64-bit counter value.
150  *
151  * Returns %0 if successful, or the following error codes:
152  *
153  * -EAGAIN      : The operation would have blocked.
154  *
155  * This is used to atomically remove a wait queue entry from the eventfd wait
156  * queue head, and read/reset the counter value.
157  */
158 int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
159 				  __u64 *cnt)
160 {
161 	unsigned long flags;
162 
163 	spin_lock_irqsave(&ctx->wqh.lock, flags);
164 	eventfd_ctx_do_read(ctx, cnt);
165 	__remove_wait_queue(&ctx->wqh, wait);
166 	if (*cnt != 0 && waitqueue_active(&ctx->wqh))
167 		wake_up_locked_poll(&ctx->wqh, POLLOUT);
168 	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
169 
170 	return *cnt != 0 ? 0 : -EAGAIN;
171 }
172 EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
173 
174 /**
175  * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
176  * @ctx: [in] Pointer to eventfd context.
177  * @no_wait: [in] Different from zero if the operation should not block.
178  * @cnt: [out] Pointer to the 64-bit counter value.
179  *
180  * Returns %0 if successful, or the following error codes:
181  *
182  * -EAGAIN      : The operation would have blocked but @no_wait was non-zero.
183  * -ERESTARTSYS : A signal interrupted the wait operation.
184  *
185  * If @no_wait is zero, the function might sleep until the eventfd internal
186  * counter becomes greater than zero.
187  */
188 ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
189 {
190 	ssize_t res;
191 	DECLARE_WAITQUEUE(wait, current);
192 
193 	spin_lock_irq(&ctx->wqh.lock);
194 	*cnt = 0;
195 	res = -EAGAIN;
196 	if (ctx->count > 0)
197 		res = 0;
198 	else if (!no_wait) {
199 		__add_wait_queue(&ctx->wqh, &wait);
200 		for (;;) {
201 			set_current_state(TASK_INTERRUPTIBLE);
202 			if (ctx->count > 0) {
203 				res = 0;
204 				break;
205 			}
206 			if (signal_pending(current)) {
207 				res = -ERESTARTSYS;
208 				break;
209 			}
210 			spin_unlock_irq(&ctx->wqh.lock);
211 			schedule();
212 			spin_lock_irq(&ctx->wqh.lock);
213 		}
214 		__remove_wait_queue(&ctx->wqh, &wait);
215 		__set_current_state(TASK_RUNNING);
216 	}
217 	if (likely(res == 0)) {
218 		eventfd_ctx_do_read(ctx, cnt);
219 		if (waitqueue_active(&ctx->wqh))
220 			wake_up_locked_poll(&ctx->wqh, POLLOUT);
221 	}
222 	spin_unlock_irq(&ctx->wqh.lock);
223 
224 	return res;
225 }
226 EXPORT_SYMBOL_GPL(eventfd_ctx_read);
227 
228 static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
229 			    loff_t *ppos)
230 {
231 	struct eventfd_ctx *ctx = file->private_data;
232 	ssize_t res;
233 	__u64 cnt;
234 
235 	if (count < sizeof(cnt))
236 		return -EINVAL;
237 	res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt);
238 	if (res < 0)
239 		return res;
240 
241 	return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
242 }
243 
244 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
245 			     loff_t *ppos)
246 {
247 	struct eventfd_ctx *ctx = file->private_data;
248 	ssize_t res;
249 	__u64 ucnt;
250 	DECLARE_WAITQUEUE(wait, current);
251 
252 	if (count < sizeof(ucnt))
253 		return -EINVAL;
254 	if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
255 		return -EFAULT;
256 	if (ucnt == ULLONG_MAX)
257 		return -EINVAL;
258 	spin_lock_irq(&ctx->wqh.lock);
259 	res = -EAGAIN;
260 	if (ULLONG_MAX - ctx->count > ucnt)
261 		res = sizeof(ucnt);
262 	else if (!(file->f_flags & O_NONBLOCK)) {
263 		__add_wait_queue(&ctx->wqh, &wait);
264 		for (res = 0;;) {
265 			set_current_state(TASK_INTERRUPTIBLE);
266 			if (ULLONG_MAX - ctx->count > ucnt) {
267 				res = sizeof(ucnt);
268 				break;
269 			}
270 			if (signal_pending(current)) {
271 				res = -ERESTARTSYS;
272 				break;
273 			}
274 			spin_unlock_irq(&ctx->wqh.lock);
275 			schedule();
276 			spin_lock_irq(&ctx->wqh.lock);
277 		}
278 		__remove_wait_queue(&ctx->wqh, &wait);
279 		__set_current_state(TASK_RUNNING);
280 	}
281 	if (likely(res > 0)) {
282 		ctx->count += ucnt;
283 		if (waitqueue_active(&ctx->wqh))
284 			wake_up_locked_poll(&ctx->wqh, POLLIN);
285 	}
286 	spin_unlock_irq(&ctx->wqh.lock);
287 
288 	return res;
289 }
290 
291 static const struct file_operations eventfd_fops = {
292 	.release	= eventfd_release,
293 	.poll		= eventfd_poll,
294 	.read		= eventfd_read,
295 	.write		= eventfd_write,
296 	.llseek		= noop_llseek,
297 };
298 
299 /**
300  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
301  * @fd: [in] Eventfd file descriptor.
302  *
303  * Returns a pointer to the eventfd file structure in case of success, or the
304  * following error pointer:
305  *
306  * -EBADF    : Invalid @fd file descriptor.
307  * -EINVAL   : The @fd file descriptor is not an eventfd file.
308  */
309 struct file *eventfd_fget(int fd)
310 {
311 	struct file *file;
312 
313 	file = fget(fd);
314 	if (!file)
315 		return ERR_PTR(-EBADF);
316 	if (file->f_op != &eventfd_fops) {
317 		fput(file);
318 		return ERR_PTR(-EINVAL);
319 	}
320 
321 	return file;
322 }
323 EXPORT_SYMBOL_GPL(eventfd_fget);
324 
325 /**
326  * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context.
327  * @fd: [in] Eventfd file descriptor.
328  *
329  * Returns a pointer to the internal eventfd context, otherwise the error
330  * pointers returned by the following functions:
331  *
332  * eventfd_fget
333  */
334 struct eventfd_ctx *eventfd_ctx_fdget(int fd)
335 {
336 	struct file *file;
337 	struct eventfd_ctx *ctx;
338 
339 	file = eventfd_fget(fd);
340 	if (IS_ERR(file))
341 		return (struct eventfd_ctx *) file;
342 	ctx = eventfd_ctx_get(file->private_data);
343 	fput(file);
344 
345 	return ctx;
346 }
347 EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
348 
349 /**
350  * eventfd_ctx_fileget - Acquires a reference to the internal eventfd context.
351  * @file: [in] Eventfd file pointer.
352  *
353  * Returns a pointer to the internal eventfd context, otherwise the error
354  * pointer:
355  *
356  * -EINVAL   : The @fd file descriptor is not an eventfd file.
357  */
358 struct eventfd_ctx *eventfd_ctx_fileget(struct file *file)
359 {
360 	if (file->f_op != &eventfd_fops)
361 		return ERR_PTR(-EINVAL);
362 
363 	return eventfd_ctx_get(file->private_data);
364 }
365 EXPORT_SYMBOL_GPL(eventfd_ctx_fileget);
366 
367 /**
368  * eventfd_file_create - Creates an eventfd file pointer.
369  * @count: Initial eventfd counter value.
370  * @flags: Flags for the eventfd file.
371  *
372  * This function creates an eventfd file pointer, w/out installing it into
373  * the fd table. This is useful when the eventfd file is used during the
374  * initialization of data structures that require extra setup after the eventfd
375  * creation. So the eventfd creation is split into the file pointer creation
376  * phase, and the file descriptor installation phase.
377  * In this way races with userspace closing the newly installed file descriptor
378  * can be avoided.
379  * Returns an eventfd file pointer, or a proper error pointer.
380  */
381 struct file *eventfd_file_create(unsigned int count, int flags)
382 {
383 	struct file *file;
384 	struct eventfd_ctx *ctx;
385 
386 	/* Check the EFD_* constants for consistency.  */
387 	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
388 	BUILD_BUG_ON(EFD_NONBLOCK != O_NONBLOCK);
389 
390 	if (flags & ~EFD_FLAGS_SET)
391 		return ERR_PTR(-EINVAL);
392 
393 	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
394 	if (!ctx)
395 		return ERR_PTR(-ENOMEM);
396 
397 	kref_init(&ctx->kref);
398 	init_waitqueue_head(&ctx->wqh);
399 	ctx->count = count;
400 	ctx->flags = flags;
401 
402 	file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
403 				  O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
404 	if (IS_ERR(file))
405 		eventfd_free_ctx(ctx);
406 
407 	return file;
408 }
409 
410 SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
411 {
412 	int fd, error;
413 	struct file *file;
414 
415 	error = get_unused_fd_flags(flags & EFD_SHARED_FCNTL_FLAGS);
416 	if (error < 0)
417 		return error;
418 	fd = error;
419 
420 	file = eventfd_file_create(count, flags);
421 	if (IS_ERR(file)) {
422 		error = PTR_ERR(file);
423 		goto err_put_unused_fd;
424 	}
425 	fd_install(fd, file);
426 
427 	return fd;
428 
429 err_put_unused_fd:
430 	put_unused_fd(fd);
431 
432 	return error;
433 }
434 
435 SYSCALL_DEFINE1(eventfd, unsigned int, count)
436 {
437 	return sys_eventfd2(count, 0);
438 }
439 
440